Apparatus and method for introducing solutes into a stream of carrier gas of a chromatograph

ABSTRACT

A precise volume of sample liquid is injected by inserting a syringe into the vaporizing chamber of the injection assembly for a gas chromatograph by performing the entire injection including withdrawal of the syringe in such a short time that essentially none of the liquid is vaporized while in the needle.

BACKGROUND OF THE INVENTION

The amounts of different solutes contained in a solvent can generally bedetermined by a gas chromatograph if the solutes are vaporizable attemperatures not in excess of 400° C. A gas chromatograph is comprisedof means for providing a stream of carrier gas, means for introducingsolutes from a sample of the solute containing solvent into the streaman oven, a separation column within the oven that elutes the solutescontained in the stream at respectively different times, and a detectorfor indicating the respective amounts of those solutes as they elute.

An early and widely used means for introducing a sample of solvent intoa stream of carrier gas includes a syringe and a heated vaporizationchamber through which the stream is made to flow. The temperature of thevaporization chamber is set above the vaporization temperature of theleast volatile solute of interest, and a desired volume of sample liquidis injected into the needle of the syringe by operation of the plungerso as to force at least some of the liquid into the vaporizationchamber. Non-volatile components in the sample are left in the needle orare deposited on the walls of the vaporization chamber and do not enterthe column.

A number of techniques have been developed for operating the syringe,but all of them exhibit discrimination against the less volatile solutesof a sample, i.e., as the volatility decreases below a given value, theamount introduced into the stream decreases. This has led to thesuggestion that the needle be left in the vaporization chamber forseveral seconds so that the solvent and all the solutes in the needleare vaporized, but this does not reduce the discrimination to a level atwhich it can be ignored. Furthermore, the amount of solutes that reachesthe stream of carrier gas varies from injection to injection. Varioustedious calibration techniques can be used to compensate for thesephenomena. Another difficulty with these injection methods of the priorart is that the vaporization of components in the needle makes itdifficult to input sample volumes that are a small fraction of thevolume of the needle.

In order to obtain accurate results without employing time-consumingcalibration techniques, it is essential that the solutes from apredetermined volume of sample and only that volume be introduced intothe stream of carrier gas. This can be done by employing a "coldon-column injection technique" wherein the top of a "T" coupling isinserted in the separating column through which low temperature carriergas is flowing to the separation column and the sample volume of solventcontaining the solutes of interest is injected via the needle of asyringe into the carrier gas through the stem of the T. Unfortunately,however, insoluble components may be present in the sample, resulting indamage to the column. In some cases, the column must be discarded and inothers, a piece must be broken off and the remaining portionrecalibrated. In addition, this technique is only applicable to verydilute samples and it requires a time-consuming cooling of the ovenbetween injections.

These matters are discussed in an article by K. Grob Jr. and H. P.Neukom, entitled "The Influence of the Syringe Needle on the Precisionand Accuracy of Vaporizing GC Injections" which was published duringJanuary 1979 in the Journal of HPC and CC at page 15 and in an articleby K. Grob and S. Rennhard, entitled "Evaluation of Syringe HandlingTechniques for Injection into Vaporizing GC Injectors" which waspublished during December 1980 in the same journal at page 627.

BRIEF SUMMARY OF THE INVENTION

In accordance with this invention, the steps of inserting the syringeneedle into the vaporization chamber, operating the plunger of thesyringe and withdrawing the needle from the vaporization chamber are alldone in such a short time that the needle does not get hot enough tovaporize a significant amount of the solvent or of any solute. Thus, asample volume leaves the needle as a liquid, the vaporizable componentsthereof assume gaseous form in the vaporization chamber so as to enterthe stream of carrier gas, the non-vaporizable components remain in theneedle or the chamber, no components are vaporized from the solventsolution that pushes the sample volume out of the needle, and there islittle or no discrimination.

It has been found that the performance described above can be attainedfor solvents such as hexane that boil at 69° C. by making the total timethat the top of the needle dwells in the vaporization chamber 500milliseconds or less. For solvents that boil at lower temperatures, thedwell time should be reduced. Satisfactory operation is attained whenpentane is used as a solvent if the dwell time is 76 milliseconds.

A significant advantage of this invention is the fact that the solutesthat only come from sample volumes that are a fraction of the needlevolume may be introduced into the stream of carrier gas. This ispossible because the purely hydraulic injection of the sample volumeinto the vaporization chamber can be precisely controlled and nothing isadded by vaporization as in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-section illustrating apparatus for performingthe insertion and withdrawal of the needle as required;

FIG. 2 is a timing diagram for the insertion and withdrawal of theneedle as well as for the injection;

FIG. 3 contains a series of graphs that respectively indicate thediscrimination resulting from different injection techniques; and

FIG. 4 is a graphical comparison of "on column" injection and injectionin accordance with this invention insofar as discrimination isconcerned.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, which illustrates one embodiment of the invention, Oindicates a portion of the top of the oven of a gas chromatograph and VCindicates a vaporization chamber extending therethrough. Thevaporization chamber VC includes a hollow cylinder 2 that is coupled viaa tube 4 to a source 6 of pressurized carrier gas and a tube 8 that iscoaxially mounted within the cylinder 2 and extends through the bottom bthereof. If sample splitting is desired, an opening 7 is provided in thecylinder 2 near the bottom b. The bottom of the tube 8 is coupled via anut 10 and packing 12 to a separating column C that extends into thetube 8. A short section of another tube 14 is coaxially mounted to thetop t of the cylinder 2 so as to communicate therewith. The top of thetube 14 has an inner recess 16 in which a septum S of perforablerubber-like material is mounted. The septum S extends above the upperend of the tube 14 and into a recess 18 in a nut 20 having inner threadsengaging threads on the outside of the tube 14. A stream of carrier gasflows through the tube 4 into the cylinder 2, up to the top of the tube8, and downwardly in the tube 8 to the column C as indicated by thearrows.

A U-shaped bracket comprised of vertical side members 22 and 24 and ahorizontal top member 26 is attached to the top portion O of the oven sothat the horizontal member 26 is spaced therefrom. Although not shown,all of these members have a T-shaped cross-section and are oriented withtheir respective stems s₂₂, s₂₄ and s₂₆ facing inwardly. A carriagecomprised of a horizontal arm 28 and a vertical arm 30 is slidablymounted on the vertical side members 22 and 24. In this particularillustration, the end of the horizontal arm 28 has a slot s₂₈ thereinthat fits snugly over the stem s₂₂ of the vertical member 22; and thevertical arm 30 has a slot s₃₀ along its outer edge that fits snuglyover the stem s₂₄ of the of the vertical member 24. Thus, the stems s₂₂and s₂₄ of the vertical members 22 and 24 form rails or guides alongwhich the carriage 28, 30 can travel in a vertical direction.

The amount and direction of travel is controlled in the followingmanner. A stepping motor M₁ having a pulley 32 mounted to one end of itsshaft is attached by a bracket 34 to the vertical member 22, and apulley 36 is suspended from the horizontal member 26. A belt 38 that isentrained over the pulleys 32 and 36 is clamped to the horizontal arm 28of the carriage at 38 so as to cause it to be moved up or down dependingon the direction of rotation of the stepping motor M₁.

A hollow cylindrical barrel portion B of a syringe having a reservoirtherein and a hollow needle N extending axially downward therefrom ismounted by brackets 40 and 42 to the vertical arm 30 of the carriage 28,30. A plunger P is mounted in the barrel portion B of the syringe with aslide fit that is tight enought to permit it to move easily and yet drawliquid into the needle N and the barrel B when it is moved upward. Apulley 44 is mounted for rotation on an axle that is affixed to abracket 46 extending downwardly from the horizontal arm 28 of carriage28, 30 and a DC motor M₂ that is mounted on the horizontal arm 28 drivesa pulley 48. A belt 50 is entrained over the pulleys 44 and 48 and isclamped to the top of the plunger P at 52 so that the plunger is movedup or down depending on the direction of rotation of the motor M₂. Themotors M₁ and M₂ may be controlled in any suitable manner or by amicroprocessor 54.

The solution to be analyzed may be located in a cup 56 that is mountedon the end of a bar 58 that can be slid in either direction as requiredby hand or by connecting it to an automatic or semi-automatictranslation device (not shown) of well known type by obvious techniqueswithin a horizontal hole 60 in a vertical bar 62 that is secured to thetop O of the oven. In more sophisticated equipment, a number of samplecups could be carried at circumferentially displaced points on a traythat is rotated in a horizontal plane about a vertical axis so as tobring the cups in line with the needle N. A U-shaped opening is providedin the outer edge of the tray that can be rotationally positioned so asto permit the barrel B to pass through it when an injection is to bemade. If desired, the rotation of the tray can also be controlled by themicroprocessor 54.

In operation, a sample is first loaded into the barrel B of the syringe.The cup 56 is positioned under the needle N and the barrel portion B islowered by the stepping motor M₁ until the tip of the needle N isimmersed in the solution. The motor M₂ then operates to raise theplunger P so as to draw solution into the barrel B. The amount in thebarrel B depends on the volume of the sample to be injected and can thanbe less or greater than the volume of the needle N. When this is done,the motor M₁ raises the barrel B of the syringe and the cup 56 is movedfrom under the needle N.

When sample fluid is to be introduced into the stream of carrier gas,the motor M₁ rapidly lowers the carriage 28, 30 until the bottom of thebarrel B approaches the nut 20 at which point the top of the needle N isat a level indicated by a dashed line 64 in FIG. 1.

FIG. 2 illustrates the various steps of an injection procedure. Duringthe period t_(d), the tip of the syringe needle N is moving downwardfrom the septum S. Note that the dashed line 64 is above the top of thetube 8. This is to make it possible for nonvolatile components to strikethe inner walls of the vaporization chamber where they will be retainedand prevented from reaching the stream of carrier gas denoted by thearrows. The tip of the needle N remains in the lowest position during aperiod t_(i) and the motor M₂ lowers the plunger P until it hasdisplaced a desired sample volume from the end of the needle N. During aperiod t_(w), the motor M₁ withdraws the barrel B and the needle N untilthe tip of the needle N is just above the bottom of the septum S. Itwould be possible to lower the plunger P during portions of either orboth of t_(d) and t_(w) in addition to t_(i). It would also be possiblefor the motor M₁ to immediately start raising the barrel B as soon as itreaches its lowest point. Another variation would be to raise theplunger P as soon as it has expelled the sample volume of liquid. FIG. 2indicates that the velocity with which the syringe is lowered and raisedare the same and constant, but this too can be varied. Whichever ofthese combinations are used, it is important that there be as littleevaporation of solutes from the needle during the entire dwell time oft_(d) +t_(i) +t_(w). Experience has shown that when hexane is used asthe solvent, good results are attained if the dwell time t_(d) +t_(i)+t_(w) is 500 milliseconds or less because hexane which is a typicalsolvent of high volatility, is not vaporized. If more volatile solventssuch as pentane are used, it is preferable to reduce the dwell time to apoint well below 500 milliseconds.

FIG. 3 shows graphically the results attained by several injectionmethods of the prior art when normalized on the nonane peak with thetemperature of the vaporization chamber at 350° C. and a one-microlitersample injected with a 15:1 split. The amount of the alkane having aC-number of 44 that is introduced into the stream of carrier gas in only25% of the nonane that is introduced when the full needle method ofinjection, which is the one generally used by automatic injection means,is employed. Whereas the percentage improved to 75% for the best priorart method of injection wherein the solvent is drawn into the barrel andthe needle heated in the vaporization chamber prior to injection, thisstill requires the use of calibration techniques. The fact that thesegraphs all converge at 100% for nonane and the fact that all the nonanein a sample volume displaced by the plunger of a syringe is introducedinto the stream of carrier gas by the "on-column" method does not meanthat the other methods do the same because all of these measurements arerelative. When using these other methods, one must use a calibrationtechnique that can compensate for the fact that the solutes actuallyintroduced into the stream of carrier gas can come from a volume ofsolvent solution that is different and usually greater than the samplevolume and which can also compensate for the discrimination.

FIG. 4 shows a comparison of the discrimination encountered when using afull needle injection having the usual dwell time with an injectionhaving the short dwell time of this invention as well as thediscrimination encountered with on-column injection. The results arenormalized on an alkane having a C-number of 20, the sample volumedisplaced by the plunger is one microliter, the temperature of thevaporization chamber was set to 350° C., and the solvent was hexane.Note that the prior art method having a dwell time of three secondsintoduces relatively more of the alkane having C-numbers less than 20into the carrier gas stream than it does of C-20, even though theamounts of all alkanes in the test sample were the same.

In these graphs, all alkanes are respectively normalized to the alkanesin the on-column injection so that it is considered to be 100%. Theimportant fact is that the discrimination resulting from using the shortdwell time of this invention is within ±3% of that attained by theon-column method so as not to require calibration. Furthermore, thesolutes introduced into the stream of carrier gas are all of those inthe sample volume and no more, so that compensation for variable volumesis not required.

What is claimed is:
 1. A method of injecting sample liquid into avaporization chamber, comprising the steps ofdrawing sample liquid intoa syringe, inserting a needle of the syringe into a heated vaporizingchamber for a period that is less than 500 miliseconds, and operatingthe syringe so as to inject a sample volume of liquid into thevaporizing chamber while the needle is inserted therein.
 2. Apparatusfor injecting sample liquid into a vaporization chamber, comprisingasyringe having a hollow needle projecting from a reservoir, meansoperable to cause said syringe to draw sample liquid into said needleand said reservoir, means operable to cause said syringe to expel liquidfrom said needle, said vaporizing chamber having three openings therein,one of said openings being closed by a septum, a second of said openingsbeing attachable to a separating column, and a third of said openingsbeing attachable to a source of carrier gas under pressure whereby whensaid latter attachment is made a stream of carrier gas flows throughsaid chamber between said second and third openings, means for movingthe syringe so as to cause the tip of its needle to pierce said septumand enter said vaporizing chamber and for subsequently withdrawing saidneedle from said vaporizing chamber within 500 milliseconds, and meansfor causing said syringe to eject liquid through said needle while thetip thereof is inside said vaporizing chamber.
 3. A method ofintroducing a sample volume of a solute containing solvent into avaporization chamber of an injection means for a gas chromatographcomprising the steps ofinserting a needle of a syringe into thevaporization chamber, operating the syringe before components in theneedle are vaporized so as to hydraulically inject a sample volume ofsolvent into the chamber in liquid form, and withdrawing the needle fromthe vaporization chamber before a significant amount of any componenttherein is vaporized.
 4. A method as described in claim 3 in which thetip of the needle is in the vaporization chamber for no more than 500milliseconds.
 5. A methed of introducing a sample volume of a solutecontaining solvent into a vaporization chamber of an injection means fora gas chromatograph comprising the steps ofinserting a needle of asyringe into said vaporization chamber, withdrawing the needle from thechamber, operating the syringe while the needle is in the vaporizationchamber so as to inject a sample volume into the chamber, all of thesaid steps being performed in no more than 500 milliseconds.